I met a wonderful man at a clinic that loaded my troublesome QH in 45 minutes. Prior to that, he had gone without one foot on the trailer for nearly 4 years. As a colt he was transported from PA to Maine, in what must have been a traumatic fashion, because he would have rather taken any punishment than set foot on a trailer. I tried EVERYTHING. He went for days without eating because we tried to feed him in it. We stood outside for hours trying to get him to load with no place to go with no success. We even resorted to butt ropes and lip chains, unfortunately, again with no results. This guy did it in short order and with very little stress to the horse or himself.
What he did made absolute sense to me. The idea is to make the trailer the relaxing place and the outside of the trailer a place to work. He put my QH in a rope halter and on a long lead. First he did some in hand ground work with him to gain respect. He lunged him outside the trailer until he felt he had his attention. Then, standing outside the trailer, on the side of the door, he allowed my horse the opportunity to load. The horse didn't. So it was back to lunging. After a few more minutes, another opportunity. Again, he refused. After some more working, my QH was in a good sweat. He offered him another opportunity and this time he took it. He hopped right on. The man did not try to tie or close him in, he just let him stand. After probably 5 minutes, he backed him off, made one lunge circle with him, gave him the opportunity to go on and he loaded right back up. He did this a few times, with variations on the time he lunged, how he approached the trailer, etc. Eventually, when the man felt that my horse was calm enough, he closed the butt bar and the back door and let him chill in the trailer for a bit. After 10 minutes or so, backed him off and that was that.
Ever since, we've never had a problem getting him on or off a trailer. In fact, we can stand outside and he loads himself. He backs off on his own, too. We've successfully trailered him in straight loads, slant loads, step-ups, ramps, stock trailers and enclosed ones with no problems.
I suggest you try this method. First, make sure your trailer is in proper condition and your horse is equipped with protective gear to keep from getting banged up. I also suggest you get a STEP UP trailer to try it with first. As Kevin, the trainer put it, you're either in or out on a step up. With a ramp, you can build resistance.